1. Field of the Invention
The present invention relates to a method of producing an epitaxial substrate for a solid-state imaging device, and a method of producing a solid-state imaging device, and to a technology that is capable of forming gettering sites in an epitaxial substrate for a solid-state imaging device easily within a short time.
Priority is claimed on Japanese Patent Application No. 2009-136363, filed Jun. 5, 2009, the content of which is incorporated herein by reference.
2. Description of Related Art
Many cellular telephones, digital video cameras or the like are equipped with high-performance solid-state imaging devices utilizing semiconductors. Recently, performance (e.g., numbers of pixels, sensitivity or the like) of these imaging device is remarkably improved. For example, the solid-state imaging devices are produced using epitaxial substrates formed by growing epitaxial layers on a surface of a semiconductor substrate, and forming a circuit of a photo-diode or the like in the epitaxial layers.
Dark leakage current of a photo-diode is a problematic factor which causes deterioration of imaging properties of a solid-state imaging device. It is considered that the dark leakage current is caused by heavy-metal contamination of a substrate (wafer) during the production process of the device. Conventionally, heavy-metal contamination of a substrate has been suppressed by forming gettering sinks for gettering heavy metals in the interior or on the back surface of a semiconductor wafer. Suppression of heavy-metal concentration in a portion of forming a photo-diode has also been performed by forming gettering sinks in a semiconductor wafer and accumulating (trapping) heavy metals by the gettering sinks.
For example, formation of gettering sinks has been performed by forming oxygen precipitated portions (oxide precipitates) within a substrate by heat treatment of the substrate, and utilizing the oxygen precipitated portions as the gettering sinks (e.g., see M. Sano, S. Sumita, T. Shigematsu and N. Fujino, Semiconductor Silicon 1994, eds. H. R. Huff et al., Electronchem. Soc., Pennington, 1994). Alternatively, gettering sinks may be formed on a back-side of a substrate by forming an amorphous film on a back surface of a substrate (e.g., Japanese Unexamined Patent Application, First Publication, No. Hei6-338507).
However, in the method of forming oxygen precipitated portions within a substrate by a heat treatment of the substrate, long time heat treatment is required for forming oxide precipitated portions of sufficient sizes to sufficiently getter the heavy metals, resulting in lengthy production process and a high production cost. In addition, there is a possibility that the heat treatment process itself may cause additional contamination of the substrate with heavy metals originating from a heating apparatus or the like.
On the other hand, in the recent stream, double-side polished large sized substrates such as 300 mm wafers are mainly used in device production. It is difficult to form gettering sinks constituted of amorphous film formed on the back surface of the substrate in the case of using double side polished substrates.
Based on the consideration of the above-described circumstance, an object of the present invention is to provide a method of producing an epitaxial substrate for a solid-state imaging device, wherein gettering sinks can be formed easily within a short time while avoiding heavy-metal contamination during the production process of the gettering sinks.
Another object of the present invention is to provide a method of producing a solid-state imaging device at low cost while suppressing heavy-metal contamination.